Efforts in the design of integrated circuits for radio frequency (RF) communication systems generally focus on improving performance, reducing cost or a combination thereof. One area of increasing interest relates to conversion of signals, such as from analog-to-digital or digital-to-analog. Both types of conversion have benefited from the development and use of delta-sigma modulation.
Delta-sigma modulation is a technique used to generate a very high fidelity (e.g., low noise) estimate of a signal using a small number of quantization levels and a very high sampling rate. Limiting a signal to a finite number of levels introduces significant “quantization” noise into the system. Oversampling and the use of an integrator feedback-loop in delta-sigma modulation are effective in shifting noise, primarily quantization noise, to out-of-band frequencies. The noise shifting properties enable efficient use of subsequent filtering stages to remove noise and produce a more precise representation of the input.
Using a quantizer with a sample rate much higher than the Nyquist frequency, an analog signal can be sampled numerous times before it can change appreciably. A difference element within the delta-sigma determines a difference between a signal sample and a feedback signal. The determined difference is then passed to an integrator. The negative feedback provided by the feedback signal causes the output of the integrator will be responsive to the degree that its average output over a given period exceeds or falls short of the signal value. The output of the integrator is then quantized, and a feedback signal based on the result is provided to correct the integrator value. A filter can recover the signal value by averaging the quantizer output over a number of cycles. This average will approach the received signal as the sampling rate increases.
It will be appreciated that the feedback signal can be weighted to be somewhat larger than the original signal. This can result in rapid adjustments of the average, causing it to oscillate quickly around the signal value over the course of the averaging process. As the average converges, the magnitude of the swings decreases, but the frequency remains high. Thus, the quantization noise becomes a high frequency oscillation around the average. This high frequency quantization noise can be filtered out to achieve a desired representation of signal having a high dynamic range.
The delta-sigma ADC offers very low quantization noise using quantizers with as small as one bit (e.g., 2 quantization levels). The smaller the quantizer the faster the sample rate to provide a level of performance. To provide very high dynamic range with small, inexpensive quantizers the ADC must be clocked at a very high rate which may require expensive technologies such as Silicon Germanium (SiGe) or Indium Phosphide (InP). Increasing the quantizer size enables performance at lower rates and can avoid the use of more expensive circuit materials but introduces or increases non-linearities in the quantization step and in the feedback converter. These non-linearities can be the limiting factor in ADC performance.